The present invention relates to a developing agent used in an image forming apparatus such as an electrostatic copying machine or a laser beam printer.
The heat roll fixing method is advantageous over other fixing methods in that it is possible to obtain a strong fixed image at high speed, in that the energy conversion efficiency is high, and in that the detrimental effect on the environment by the evaporation of, for example, solvent is small.
On the other hand, it has been pointed out that the heat roll fixing method gives rise easily to a so-called “low-temperature offset” phenomenon and a so-called “high-temperature offset” phenomenon, in which the toner image is brought into direct contact with the fixing roll or the belt at a low or a high temperature.
The low-temperature offset phenomenon represents the phenomenon that the toner image is not sufficiently melted at a low temperature and, thus, the toner is cracked when the toner image is brought into contact with the fixing roller. On the other hand, the high-temperature offset phenomenon represents the phenomenon that the toner is excessively melted at a high temperature so as to cause the molten toner to be attached to, for example, the fixing roll.
It is desirable for the low-temperature offset phenomenon to be generated under a lower temperature and for the high-temperature offset phenomenon to be generated under a higher temperature. Along this line, various ideas have been proposed to date. As some of these proposals, it is known in the art to broaden the molecular weight distribution of the binder resin and to add as a releasing agent a low-molecular-weight polypropylene series resin having a prescribed range of molecular weight distribution to the binder resin. In addition, it is proposed to add two kinds of wax component to the toner in order to obtain a toner excellent in low-temperature fixing properties and in resistance to the offset phenomenon.
On the other hand, in view of energy saving, it is desirable to shorten the waiting time until the fixing device is heated to the temperature at which the fixing device is operated and to carry out the fixing at a lower temperature. As a method for lowering the fixing temperature, it is known in the art to use a toner containing a crystalline polyester and an amorphous polyester resin as a binder resin, as disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-222138.
However, even the toner referred to above fails to satisfy all the required performances so as to give rise to some problems. To be more specific, the particular toner is incapable of satisfying both the resistance to the offset phenomenon at a low temperature and the resistance to the offset phenomenon at a high temperature. For example, toner which is excellent in its resistance to the offset phenomenon at a high temperature and in its developing properties is insufficient in its fixing properties at a low temperature. In contrast, toner which is excellent in its resistance to the offset phenomenon at a low temperature and in its fixing properties at a low temperature is somewhat poor in its resistance to blocking and in its developing properties.